Electrochemical fuel cells convert fluid reactants, namely a stream of fuel and a stream of oxidizer, into electric power, heat and reaction products. Fuel cells generally comprise an electrolyte arranged between two porous electrically conductive electrode layers. In order to induce the desired electrochemical reaction, the anode electrode and the cathode electrode can each comprise one or more catalyst.
At the anode, the fuel moves through the porous electrode layer and is oxidized by the catalyst to produce both cations (generally protons) and electrons. The cations migrate through the electrolyte towards the cathode. The oxidizer (generally pure oxygen or a mixture containing oxygen) moves through the porous cathode and reacts with the cations coming through the membrane from the anode. The electrons, for their part, travel from the anode to the cathode through an external circuit, producing an electrical current. The electrochemical reaction between the oxidizer and the cations also produces heat. The heat causes the temperature of the fuel cell to increase during operation.
In order to produce a substantial amount of electrical power, a number of individual fuel cells must be stacked together in series with an electrically conducting separator between each fuel cell. Such an assembly of fuel cells stacked together is known as a fuel cell stack. For applications requiring a lot of energy, such as powering a vehicle for instance, very large fuel cell stacks are often used. These large stacks can sometimes comprise a hundred fuel cells or more. As would be expected, the fuel cell stacks produce quite a large amount of heat and need to be cooled in order to maintain an optimal operating temperature.
One common way of cooling a fuel cell stack is to circulate a cooling fluid through channels defining cooling circuits inside the stack. In most cases, the cooling fluid is liquid, preferably a mixture of water and anti-freeze like ethylene glycol with the possible addition of corrosion inhibitors or other agents. Patent document US 2003/0203258 discloses a prior art coolant circulation arrangement for a fuel cell stack. In a known manner, the circulation arrangement comprises a reservoir for supplying cooling liquid, a pump for driving the cooling liquid from the reservoir through channels inside the fuel cell stack, and a radiator for removing heat from the cooling liquid returning to the reservoir.
The use of a coolant circulation arrangement like the one just described allows maintaining the temperature of the fuel cells in a stack at, or near, their optimal operating temperature. Although different types of fuel cells have different optimal operating temperatures, this temperature is practically always substantially above the surrounding ambient temperature. One consequence of the temperature difference existing between the inside of the stack and the surroundings is that the reactant gases supplied to the fuel cells can be substantially colder than the fuel cells. Such a temperature difference between the reactant gases and the fuel cells can cause a number of problems. In particular, the input of colder gas into the fuel cells creates detrimental temperature differences inside the fuel cells. Furthermore, a temperature difference between stack and reactant can also lead to unwanted condensation of water vapour, in particular when recycled off gas is mixed with the reactant gas. It follows from the above that the temperature of the reactant gases supplied to the fuel cells should preferably be preheated to approximately the same temperature as the inside of fuel cell stack.
In order to preheat the reactant gases, it is known to bring them into thermal contact with the cooling fluid exiting form the fuel cell stack. For this purpose, the previously mentioned prior art coolant circulation arrangement disclosed in patent document US 2003/0203258 comprises a heat exchanger for exchanging heat between the cooling liquid and the storage container for the fuel gas. One problem with this arrangement arises from the fact that the canisters used for storing the fuel supply are arranged apart from the fuel cell stack. It follows that, as the heat exchanger has to be in direct contact with the canisters, it is necessary to provide ducts for delivering the cooling liquid from the stack to the heat exchanger. The presence of ducts for the cooling liquid contributes to increase the cost of making the fuel cell system. Furthermore, the system also comprises supply ducts to lead the preheated fuel gas to the stack. One will understand that, with such an arrangement, it is necessary to provide the supply ducts with thermal insulation in order to prevent the preheated fuel gas from cooling before it reaches the fuel cells. The presence of thermal insulation also contributes to increasing the cost of making the fuel cell system.